This invention relates to complementary amplifier circuitry and more particularly to circuitry for biasing a push-pull stage to substantially eliminate crossover distortion.
Non-Complementary-Transistor push-pull amplifiers traditionally comprise a pair of like conductivity output transistors with their principal conduction paths connected in series between supply potentials. The control electrodes of these transistors are driven antiphase by a class A amplifier in such a fashion that only one of the output transistors conducts at a time to minimize power dissipation in the amplifier. These amplifiers tend to distort the signal at the output potential level where one of the output transistors is turning on and the other is turning off. To overcome this problem the output transistors may be biased so that in the quiescent state, i.e., at the crossover potential, both transistors remain in conduction. This, however, generally requires that the stage driving the output transistors be biased to pass significant current resulting in added power dissipation.
A. F. Petrie in U.S. Pat. No. 3,454,889 describes a push-pull stage (FIG. 1) wherein the output transistors are driven by a phase splitter including a bipolar transistor (npn) having a first resistor connected in its emitter circuit and a second resistor connected in its collector circuit. The control electrode of the pull down (npn) output transistor is connected to the emitter electrode of the driver transistor and the control electrode of the pull up output transistor (npn) is connected to the collector electrode of the driver transistor. In addition, a diode is connected between the emitter electrode of the pull up transistor and the collector electrode of the driver transistor poled to conduct in the direction toward the collector of the driver transistor. This arrangement tends to constrain the collector potential of the driver transistor from falling below some minimum potential, the effect of which provides extra base current drive to the pull down output transistor. However, the potential drop across the diode also reverse biases the base-emitter junction of the pull up output transistor turning it hard off which in turn contributes to cross over distortion. A further shortcoming of this circuit when it is implemented in integrated circuit form is the difficulty of controlling the absolute value of the resistors in the driver circuit. This results in difficulty in controlling the amplitude of the idling current conducted by the output transistors.
M. J. Teague in the article, "Get pnp Class-B Stage Efficiency" in the March 15, 1967 issue of Electronic-Design magazine (pp. 238-240) describes a non-complementary push-pull amplifier (FIG. 2) comprising a driver transistor with collector and emitter resistors, the collector and emitter of the driver respectively driving the control electrodes of the pull-up and pull-down output transistors. This circuit includes a first diode connected between the emitter electrode of the pull-up transistor and the collector electrode of the pull-down transistor and poled to conduct in the direction of the pull-down transistor. Two additional diodes are connected in series between the control electrode of the pull-up transistor and the collector of the pull-down transistor. The two series diodes parallel the series connection of the base-emitter of the pull-up transistor and the first diode to form a diode bridge which establishes the idling current in the pull-up transistor. Output signal is taken from the emitter electrode of the pull-up transistor and base current to the driver transistor is supplied by a feedback resistor connected between the output terminal and the base electrode of the driver transistor. In addition to supplying base current to the driver transistor, the feedback resistor is necessary to stabilize the small signal voltage gain over varying output voltage levels.
There are two obvious shortcomings of this circuit. The first is that bias current for the pull-up transistor is regulated by a resistor which tends to make the output standby current dependent upon the no load output potential. The second is that base drive to the driver stage is limited by the feedback resistor which operates in a degenerative mode, i.e., when the pull-down transistor is on, the output potential is low so that little base current is provided through the feedback resistor to the emitter-follower driver. Since the inherent capacitance at the control electrode of the driver transistor must be charged and discharged by signal current through the feedback resistor, the presence of the feedback resistor tends to reduce the bandwidth of the amplifier.
A further shortcoming of the circuit is that the imposition of the diode between the output terminal and the pull down transistor tends to increase the output impedance of the amplifier.